The invention relates generally to the routing of connections in a telecommunications network.
The ATM (Asynchronous Transfer Mode) network is a data transmission system where data is transferred in digital form as 53-byte cells from one terminal to another through switches and rapid transfer connections therebetween. Each cell contains a 48-byte payload and a 5-byte header. In order to save space, the header information of each cell does not include complete address information describing the used data transfer route from the transmitting to the receiving device, but only information of the virtual path and channel where said data transfer connection is being carried. The switches or nodes of the network contain necessary routing information, on the basis whereof said identifiers of the virtual path and channel are interpreted as references to the respective node in succession.
It is to be expected that in the future the ATM network, which has so far been based mainly on cable connections, will also serve wireless terminals that are in contact with the network via radio base stations, i.e. access points. These terminals may move with respect to the base stations and their coverage area, in which case the system must be able to execute a handover when necessary. A specific feature of each ATM connection is the contract between the wireless terminal and the network as to the quality of service (QoS) required by the connection. This contract particularly covers the maximum length of the delays allowed in the connection and the highest allowed probability of cell loss. The agreed quality of service for the connection and its upkeep are important factors when making decisions as for the point of time of the handover and the new base station to be assigned for a given wireless terminal.
The PNNI protocol developed for the routing of a fixed ATM network defines how the switches of the ATM network can discover the network structure and transmit structural information to each other. The PNNI protocol also defines the routing method whichxe2x80x94on the basis of said structural data, the offered traffic and the required quality of service for the connection, as well as the available resourcesxe2x80x94finds the most advantageous route to the new connection.
FIG. 1 illustrates an example of a network structure according to the PNNI protocol. According to the PNNI protocol, the switches or nodes of the ATM network are grouped into peer groups, and one of the nodes in the group serves as the peer group leader. In FIG. 1, the peer group leaders are marked with black circles. A peer group is formed by such nodes that have a common ancestor in the PNNI address and routing hierarchy. The PNNI network structure is multi-layered: the peer group of one level forms, on the successive higher level, one logical node. The nodes of the lowest level are physical switches of the ATM network. In the PNNI protocol, each node only knows the structure of its own peer group and of those higher-level peer groups to which the peer group of said node belongs.
In the PNNI protocol, each node maintains a database as for the structure of its own peer group and the connections of said peer group with other adjacent peer groups. For this purpose, the nodes transmit, at given intervals, information of their activity and their connections with other nodes via PTSE packages. Thus each node and peer group has real-time information of the network structure, so that for instance in the case of malfunction, the peer group is capable of changing the routing of connections past the damaged connection or node.
In FIG. 1, one peer group is formed for instance of the nodes A.1.1-A.1.5. On the next higher level, this peer group A.1 is represented by the logical node A.1. The nodes A.2.1-A.2.4 form another peer group A.2. On the higher level the logical nodes A.1-A.4, each of which represents a given lower-level peer group, form the peer group A.
For the sake of clarity, all nodes of all peer groups are not individually specified in FIG. 1. Such lines that in the illustrations of the present applicationxe2x80x94particularly in FIGS. 1 and 2xe2x80x94are attached to the node at one end only represent connections directed to outside the part of the network included in the drawing. The PNNI protocol is described in more detail for instance in the publication xe2x80x9cPrivate Networkxe2x80x94Network Interface Specification Version 1.0xe2x80x9d by The ATM Forum.
A tree topology has clear advantages in realising a wireless ATM system, for instance in setting up new connections during handovers while the wireless terminal moves within the network, which is explained for instance in the publication xe2x80x9cAn Architecture and Methodology for Wireless-Executed Handoff in Cellular ATM Networksxe2x80x9d, A. S. Acampora and M. Naghshineh, IEEE Journal on Selected Areas in Communications, Vol. 12, No. 8, October 1994, p. 1365. Said publication introduces a network installed permanently in the form of a tree topology. This type of fixed tree topology has one root node, which is typically provided with a number of special functions and all connections are routed via the root node. Such a structure easily results in a non-optimal routing, which causes extra delays in the data transmission and resource losses in the data transmission network. In this type of structure, the root node also is easily overloaded.
In an embodiment of the radio part of an ATM network described above, such a tree topology is difficult to realise, because the wireless-specific switches supporting wireless terminals also participate in transmitting regular ATM connections. A regular ATM network typically has a network structure, as is illustrated in FIG. 1. To force the wireless-specific switches into a tree topology would disturb the transmission of regular telecommunications.
An object of the invention is to realise a structure whereby the above described drawbacks can be avoided. Another object of the invention is to introduce a method for facilitating routing in a network-structured data transmission network.
These objects are achieved by defining in the network structure a number of tree topologies, used in routing the connections, by arranging the switch that starts the routing to select the tree topology to be used from among a number of predetermined tree topologies, and by transmitting the identifier of the employed tree topology to all switches participating in the routing.
The data transmission network according to the inventionxe2x80x94comprising switches and data transmission connections therebetween, so that at least a given part of the switches are arranged to control the connection routing to the data transmission network according to a predetermined tree topology composed of the switches of said data transmission network and of connections therebetweenxe2x80x94is characterised in that each of said switches belonging to said at least a certain part of the switches is arranged to function as the anchor node of the tree topology during routing controlled by the switch in question.
The method according to the invention is characterised in that in said method, the switch serving as the anchor node of the tree topology controlling the routing is the switch from which the routing starts.
In the network structure according to the invention, connections are set up by using a predetermined partial group of connections constituting a tree topology. Advantageously the number of these predetermined tree topologies is larger than one, in which case the node starting the setup chooses the tree topology to be used in setting up the connection in question. Thus the advantages brought about by the tree topology can be flexibly utilised in a network-structured data transmission network.